JP5521597B2 - rice cooker - Google Patents

Description

  The present invention relates to a rice cooker using induction heating used in general households.

Conventionally, this type of induction heating type cooker includes a heating coil for induction heating of a pan, a voltage-driven type (having a gate terminal) switching element (semiconductor switching element) for turning on and off the heating coil, and the switching element. A control unit that turns on and off and a drive unit (drive circuit) that receives a signal from the control unit and drives the gate terminal of the switching element, and the drive unit applies the signal to the gate terminal of the switching element when the switching element is turned on. Some have a charging resistor that limits a charging current and a discharging resistor that discharges the charge charged in the gate terminal of the switching element when the switching element is turned off. (For example, see Patent Document 1)
With such a configuration, the power required to turn on the switching element is reduced, and the output capacity of the power supply circuit can be reduced. Further, since the accumulated charge is small when the switching element is turned off, the spot-like heat generation can be suppressed, and the destruction of the switching element due to the spot-like heat generation can be suppressed.

  However, as a result of increasing the switching speed by using such voltage-driven (having a gate terminal) switching element, the turn-off loss when the switching element is turned off has been reduced. The operation also becomes faster, and the spike current at turn-on increases. To counter this, the charging resistance of the drive unit is increased to reduce the charging current to the gate terminal.

  However, with this measure, even if the peak value of the spike current is suppressed, the voltage at the gate terminal of the switching element does not rise from the threshold voltage, the voltage between the collector and the emitter during steady-on operation increases, and the loss during steady-on The problem of increasing is generated.

  Therefore, in order to reduce steady-state on-loss, an element voltage detection unit that detects the element voltage of the switching element is provided, and impedance variable means that increases the charging resistance of the drive unit when the element voltage is higher than a predetermined value is provided. was there. (For example, see Patent Document 2)

Japanese Patent No. 2870945 Japanese Patent Laid-Open No. 11-329697

  However, in the conventional configuration, unless the charging resistance of the switching element is made variable, it is impossible to achieve both reduction in turn-on loss due to spike current when the switching element is turned on and reduction in steady on-loss when the switching element is in a steady state. It had a problem.

The present invention solves the above-described conventional problems, and includes a first DC power supply circuit for supplying power to the semiconductor switching element via a drive circuit, and supplies power to the control unit from the first DC power supply circuit. An object of the present invention is to provide an induction heating rice cooker that constitutes a second DC power supply circuit for adjusting the input voltage of the gate terminal of the semiconductor switching element according to the ON time of the semiconductor switching element.

In order to solve the conventional problems, 炊 rice apparatus of the present invention includes a heating coil for induction heating the pan, and a capacitor connected in parallel or connected in series with said heating coil, connected to the heating coil, the heating coil An inverter circuit having a semiconductor switching element that conducts and cuts off, a rectifier circuit that rectifies an AC power source and supplies power to the inverter circuit, a drive circuit that turns on and off the gate terminal of the semiconductor switching element, and the drive circuit A control unit for controlling on / off of the semiconductor switching element, a first DC power supply circuit for converting the AC power supply to a DC power supply, and a power supply voltage supplied from the first DC power supply circuit, the first DC power supply circuit It has a second DC power supply circuit to lower the output voltage voltage, the said first direct current power supply circuit, the semiconductor switching element Is configured to output at least twice the voltage than the threshold voltage of the gate terminal of the control unit is supplied with power supply voltage from the second DC power supply circuit, wherein the driving circuit, the first DC Power is supplied from the power supply circuit, and the control unit increases and adjusts the voltage output from the drive circuit to the gate terminal of the semiconductor switching element by extending the time of the ON signal to be output .

  Since the output voltage of the first DC power supply circuit that supplies power to the gate terminal of the semiconductor switching element (for example, IGBT) is configured to be larger than the threshold voltage of the gate terminal, the parasitic capacitance and current limit of the gate terminal are configured. Even if there is a resistance, the gate terminal voltage can be continuously increased from the threshold voltage.

  Therefore, when the ON time is short, the voltage input to the gate terminal of the semiconductor switching element can be reduced, and the loss of the drive circuit can be reduced during high-frequency operation. Further, the apparent impedance of the main path of the semiconductor switching element (for example, between the collector and the emitter of the IGBT) can be increased, the turn-on current when the semiconductor switching element is turned on is reduced, and the peak current of the semiconductor switching element is suppressed. And turn-on loss can be reduced.

  Next, even if the current flowing through the semiconductor switching element gradually increases due to the inductance component of the heating coil as the on-time becomes longer, the gate terminal voltage becomes almost the same as the increase amount. The apparent impedance or on-voltage of the main path of the device (for example, the path between the collector and the emitter of the IGBT) can be reduced, and the steady on-loss can be prevented from increasing.

  As described above, since the output voltage of the first DC power supply circuit is made larger than the threshold voltage of the gate terminal even if the gate terminal has a parasitic capacitance and a current limiting resistor, the gate terminal voltage gradually increases according to the ON time. The gate terminal voltage can be adjusted by adjusting the ON time. As a result, the effects as described above are exhibited, the turn-on loss can be reduced without increasing the steady-state on-loss of the semiconductor switching element, and the total loss of the semiconductor switching element can be reduced.

The rice cooker of the present invention is configured so that the first DC power supply circuit outputs a voltage larger than the threshold voltage of the gate terminal of the semiconductor switching element, and the control unit is supplied with the power supply voltage from the second DC power supply circuit, The drive circuit is supplied with power from the first DC power supply circuit, and adjusts the voltage input to the gate terminal of the semiconductor switching element by the length of time of the ON signal output from the control unit, thereby flowing the semiconductor switching element. Since the ON voltage of the gate terminal of the semiconductor switching element is adjusted according to the current and the ON voltage of the semiconductor switching element (for example, the collector-emitter voltage) is adjusted, the loss of the semiconductor switching element can be reduced.

Main part block diagram of induction heating type rice cooker in Embodiment 1 of the present invention Sectional drawing of the induction heating type rice cooker in Embodiment 1 of this invention. The main block diagram of the drive circuit of the induction heating type rice cooker in Embodiment 1 of this invention The graph which shows the on time of the semiconductor switching element (IGBT) of the induction heating type rice cooker in Embodiment 1 of this invention, and the collector current. The graph which shows the relationship between the input current supplied from the alternating current power supply of the induction heating type rice cooker in Embodiment 1 of this invention, and the collector current of a semiconductor switching element (IGBT) The graph which shows the relationship between the collector-emitter voltage of the semiconductor switching element (IGBT) of the induction heating type rice cooker in Embodiment 1 of this invention, collector current, and gate terminal voltage. Main part operation waveform diagram at start-up of induction heating rice cooker in embodiment 1 of the present invention Main part operation waveform diagram at the start-up of the conventional induction heating rice cooker Main part operation waveform diagram in the vicinity of second input current set value of induction heating type rice cooker in embodiment 1 of the present invention The time chart which shows the temperature detection part of the rice cooker in Embodiment 1 of this invention, an input electric current setting value, and the ON time of a semiconductor switching element. Main part block diagram of induction heating type rice cooker in Embodiment 2 of the present invention

The first invention is an inverter circuit having a heating coil for inductively heating a pan, a capacitor connected in parallel or in series to the heating coil, and a semiconductor switching element connected to the heating coil and conducting and blocking the heating coil. When the rectified AC power the inverter circuit to the power supply rectifier circuit, a drive circuit for turning on and off the gate terminal of the semiconductor switching element, and a control unit for turning on and off the said semiconductor switching element via the driving circuit, A first DC power supply circuit that converts the AC power supply into a DC power supply; and a second DC that is supplied with a power supply voltage from the first DC power supply circuit and has a voltage lower than the output voltage of the first DC power supply circuit. It includes a power supply circuit, the said first direct current power supply circuit, the threshold voltage of the gate terminal of the semiconductor switching elements Is configured to output at least twice the voltage, the control unit is supplied with power supply voltage from the second DC power supply circuit, the driving circuit is powered from the first DC power supply circuit, wherein, by the driving circuit by increasing the time of the output oN signal is adjusted by increasing the voltage to force out to the gate terminal of the semiconductor switching element, the gate of the semiconductor switching elements (e.g., IGBT) Since the output voltage of the first DC power supply circuit that supplies power to the terminal is configured to be higher than the threshold voltage of the gate terminal, even if there is a parasitic capacitance or current limiting resistance of the gate terminal, The gate terminal voltage can be continuously increased.

  Therefore, when the ON time is short, the voltage input to the gate terminal of the semiconductor switching element can be reduced, and the loss of the drive circuit can be reduced during high-frequency operation. Moreover, the apparent impedance between the collector and emitter of the semiconductor switching element can be increased, the turn-on current when the semiconductor switching element is turned on can be reduced, the peak current of the semiconductor switching element can be suppressed, and the turn-on loss can be reduced. it can.

  Next, even if the current flowing through the semiconductor switching element gradually increases due to the inductance component of the heating coil as the on-time becomes longer, the gate terminal voltage becomes almost the same as the increase amount. The apparent impedance of the element can be reduced, and steady on-loss can be prevented from increasing.

  As described above, since the output voltage of the first DC power supply circuit is made larger than the threshold voltage of the gate terminal even if the gate terminal has a parasitic capacitance and a current limiting resistor, the gate terminal voltage gradually increases according to the ON time. The gate terminal voltage can be adjusted by adjusting the ON time. As a result, the effects as described above are exhibited, the turn-on loss can be reduced without increasing the steady-state on-loss of the semiconductor switching element, and the total loss of the semiconductor switching element can be reduced.

A second invention is, in particular, by providing the upper limit voltage the drive circuit of 炊 rice apparatus of the first aspect of the invention is outputted to the gate terminal of the semiconductor switching elements, the absolute maximum rating of the gate terminal of the semiconductor switching elements The drive circuit can input a voltage to the gate terminal so as not to exceed the voltage. From this result, even if the output voltage of the first DC power supply voltage circuit exceeds the absolute maximum rated voltage of the gate terminal of the semiconductor switching element, the drive circuit sets an upper limit on the input voltage to the gate terminal. It is also possible to match the output voltage of the DC power supply circuit to other loads.

A third invention is, in particular first or 炊 rice device of the second invention has an input current detection circuit for detecting a value corresponding to the input current supplied to the rectifying circuit from the AC power source, the control parts has a microcomputer which stores a cooking sequence and warmth sequence, the microcomputer has a predetermined input current set value according to the thermal insulation sequence and the cooking sequence, the input current detection and the input current set value by comparing the output value of the circuit, the output value of the input current sense circuitry sets the oN time such that the input current set value can be supplied required cooking power, thermal insulation power according to each sequence .

  Further, when the input current is low, a turn-on voltage of the semiconductor switching element may be generated. At this time, since the on-time is short, the input voltage to the gate terminal of the IGBT is lowered, the apparent impedance of the main path of the semiconductor switching element (for example, between the collector and emitter of the IGBT) is increased, and the inrush current at turn-on is reduced. Thus, the turn-on loss can be reduced.

  When the ON time is long, the current flowing through the heating coil also increases, and the current of the main path of the semiconductor switching element (for example, the collector current of the IGBT) also increases. However, the gate terminal voltage increases due to the charging current charged from the first DC power supply circuit to the gate terminal of the semiconductor switching element through the drive circuit, the IGBT collector-emitter voltage gradually decreases, and the IGBT steady state ON loss can be reduced.

  That is, the loss of the semiconductor switching element is largely classified into a turn-on loss, a steady-on loss, and a turn-off loss, but the ratio of these three losses changes depending on the magnitude of the current flowing through the main path of the semiconductor switching element. Since the magnitude of the current flowing through the semiconductor switching element is substantially proportional to the on-time of the semiconductor switching element, adjusting the length of the on-time will adjust the gate terminal voltage of the semiconductor switching element, and thus semiconductor switching. The loss of the element can be reduced.

A fourth invention is, in particular first or 炊 rice device of the second invention has a voltage detection circuit for detecting a voltage corresponding to the voltage across the heating coil, the control unit and the cooking sequence warmth sequence has the stored microcomputer, the microcomputer compares the output value of the in accordance with the cooking sequence and the warmth sequence has a predetermined voltage setting value, the voltage detection circuit and the voltage set value, the voltage By setting the ON time so that the output value of the detection circuit becomes the voltage setting value, it is possible to supply necessary rice cooking power and heat retention power according to each sequence.

  Further, when the voltage corresponding to the voltage across the heating coil is low, the turn-on voltage of the semiconductor switching element may be generated. At this time, since the on-time is short, for example, the input voltage to the gate terminal of the IGBT which is an example of the semiconductor switching element is lowered, and the apparent impedance of the main path (for example, between the collector and emitter of the IGBT) of the semiconductor switching element. Increases, the inrush current at turn-on can be reduced, and the turn-on loss can be reduced.

  Further, when the on-time is long, the current flowing through the heating coil also increases, and the voltage corresponding to the voltage across the heating coil also increases. When the current flowing through the heating coil increases, the current in the main path of the IGBT which is an example of the semiconductor switching element also increases. However, the gate terminal voltage increases due to the charging current charged in the gate terminal of the semiconductor switching element from the first DC power supply circuit via the drive circuit, and the voltage between the collector and emitter of the IGBT which is an example of the semiconductor switching element gradually increases. The steady-state ON loss of the IGBT can be reduced.

  That is, the loss of the semiconductor switching element is largely classified into a turn-on loss, a steady-on loss, and a turn-off loss, but the ratio of these three losses changes depending on the magnitude of the current flowing through the main path of the semiconductor switching element. Since the magnitude of the current flowing through the semiconductor switching element is substantially proportional to the voltage across the heating coil and the on-time of the semiconductor switching element, the gate terminal voltage of the semiconductor switching element is adjusted by adjusting the length of the on-time. As a result, the loss of the semiconductor switching element can be reduced.

A fifth invention is, in particular, third or 炊 rice device of the fourth invention, the control unit has an initial value of the on-time, the initial value exceeds the threshold voltage of the gate terminal voltage of the semiconductor switching elements a value, when starting the heating coil, by setting the on-time of the semiconductor switching element to the initial value, when the turning on the semiconductor switching element immediately after starting, the gate terminal voltage of the semiconductor switching elements Since it can be started with a small size, the current flowing from the rectifier circuit through the capacitor to the main path of the semiconductor switching element can be reduced by increasing the impedance of the semiconductor switching element, and the absolute maximum current rating of the semiconductor switching element is not exceeded. Can be.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that the present invention is not limited to the embodiments.

(Embodiment 1)
FIG. 1 is a main part block diagram of a rice cooker according to the first embodiment of the present invention.

  In FIG. 1, reference numeral 1 denotes a pan, which is not particularly illustrated, and is composed of a laminated body using a plurality of magnetic metals and nonmagnetic metals. Although not shown in particular, the heating coil 2 includes a first heating coil that faces the center of the bottom surface of the pan 1 and a second heating coil that faces the corner portion of the bottom surface of the pan 1. The first heating coil and the second heating coil are electrically connected in series.

  Although not particularly illustrated, the first heating coil has a spiral shape and is arranged at a certain distance in the center of the bottom surface of the pan 1. The second heating coil is disposed on the concentric outer side of the first heating coil, and is disposed at a certain distance from the corner portion of the bottom surface of the pan 1. The heating coil 2 is constituted by a wire obtained by twisting more than 20 litz wires, each of which is a bundle of a plurality of copper wires, and makes the current distribution uniform when a high-frequency current flows.

  An inverter circuit 3 includes a capacitor 4, a semiconductor switching element 5, and a diode 6. The capacitor 4 is connected in parallel to the heating coil 2 as shown in FIG. In this embodiment, a polypropylene capacitor is used which has little loss even when a high frequency current flows. The semiconductor switching element 5 is composed of a semiconductor element such as a MOSFET or an IGBT. MOSFETs and IGBTs have high withstand voltage, can be switched at high frequency, and can flow a large current by applying a voltage to the gate terminal. Therefore, there is an advantage that a large current can be flowed with less power than a power transistor. is there. In the present embodiment, an IGBT is used for this semiconductor element.

  For example, the IGBT has a tendency that the collector current is limited by the magnitude of the voltage applied to the gate terminal. Although the collector-emitter voltage varies depending on the voltage applied to the gate terminal and the collector current, the collector-emitter voltage exceeds a certain level even when the current is lowered. This is a general characteristic of an IGBT that is a voltage-driven transistor. The relationship among the collector current of the IGBT, the collector-emitter voltage, and the voltage applied to the gate terminal will be described separately with reference to FIG.

  Also, the MOSFET tends to have a limited drain current depending on the voltage applied to the gate terminal. In the case of a MOSFET, the drain-source voltage increases in proportion to the magnitude of the drain current, but the proportionality constant varies with the voltage applied to the gate terminal. This is a general characteristic of MOSFETs. The relationship between the drain current of the MOSFET, the drain-source voltage, and the voltage applied to the gate terminal will be described separately with reference to FIG.

  The diodes 6 are connected in parallel so that a current flows in a direction opposite to the current direction of the semiconductor switching element 5. Generally, such a configuration of the inverter circuit is called a one-stone voltage resonance type inverter because the heating coil 2 and the capacitor 3 form a parallel resonance circuit.

  7 is an AC power source for supplying power to the rice cooker. The power supply frequency of the AC power supply 7 is 50 Hz in the eastern Japan region and 60 Hz in the western Japan region. Reference numeral 8 denotes a rectifier circuit, which includes a diode bridge 9, a coil 10 and a capacitor 11. Here, the capacity of the capacitor 11 is as small as several μF, and ripple occurs when a current is passed through the heating coil 2. In the present embodiment, the ripple voltage waveform is the same as the voltage waveform when the AC power supply 7 is full-wave rectified.

  The drive circuit 12 includes an NPN transistor, a PNP transistor, and a current limiting resistor for limiting the current charged in the parasitic capacitance of the gate terminal of the IGBT constituting the semiconductor switching element 5 and a discharging resistor for quickly extracting the parasitic capacitance of the gate terminal. It is comprised by the push pull circuit which has. An example of the configuration of the drive circuit 12 will be described with reference to FIG. The input current detection circuit 13 is connected to a current path from the AC power supply 7 to the rectifier circuit 8. Although not particularly shown, the current transformer and a rectifying / smoothing circuit for rectifying and smoothing an alternating current output from the current transformer are configured to convert an input current value into an analog voltage value. An analog voltage corresponding to this input current value is output to the AD conversion port of the microcomputer 15 constituting the control unit 14.

The control unit 14 includes a microcomputer 15, a synchronization signal generation circuit 16, and the like. The microcomputer 15 includes an on-time setting unit 17, a pulse generation unit 18, an input current setting unit 19, a sequence setting unit 20, and the like using a number of counter functions, timer functions, and memory functions.

  The on-time setting unit 17 sets the on-time within the range of the minimum on-time Ton1 and the maximum on-time Ton4. In the present embodiment, when the inverter circuit 3 is activated, it starts from Ton1 as an initial value and gradually increases the on-time.

  When the pulse generation unit 18 detects the synchronization signal from the synchronization signal generation circuit 16, the pulse generation unit 18 outputs a high pulse having a set on time to the drive circuit 12. The microcomputer 15 operates with an 8 MHz oscillator and a 32.728 kHz oscillator. Therefore, the minimum setting unit of the high pulse width of the pulse generator 18 composed of the microcomputer 15 is controlled in units of 0.125 μs which is the minimum period set by the 8 MHz oscillator. That is, when the on-time setting unit 17 changes the output data (8 bits) by 1 digit, the high pulse width of the pulse generation unit 18 is changed by 0.125 μs.

  The input current setting unit 19 stores a plurality of input current setting values for each sequence set by the sequence setting unit 20 using the memory of the microcomputer 15. In the induction heating type rice cooker of the present embodiment, the first input current set value 21 (in this embodiment, Iin1), the second input current set value lower than the first input current set value 21 22 (referred to as Iin2 in the present embodiment) and a third input current set value 23 (referred to as Iin3 in the present embodiment) that is lower than the second input current set value 22 are stored.

  The sequence setting unit 20 sets a rice cooking sequence or a heat retention sequence according to the operation set by the user. Moreover, a standby state is set when neither the rice cooking sequence nor the heat retention sequence is operating.

  The synchronization signal generation circuit 16 is composed of a comparator, a resistance voltage dividing circuit, and the like, and compares a voltage obtained by resistance-dividing the (C2) terminal at a predetermined ratio and a voltage obtained by resistance-dividing the (CE) terminal at a predetermined ratio. The high signal is output to the microcomputer 15 when the divided voltage at the terminal (C2) is higher, and the low signal is output to the microcomputer 15 when the divided voltage at the terminal (C2) is lower. To do.

  Although not specifically shown, the first DC power supply circuit 24 is constituted by a switching power supply, and converts the voltage obtained by half-wave rectifying the AC power supply 7 into a DC power supply so as to output a DC voltage of about 20V.

  In the present embodiment, the guaranteed variation value of the cutoff voltage (threshold voltage) of the gate terminal of the IGBT constituting the semiconductor switching element 5 is 6V. That is, when a voltage of 6 V or higher is turned on at the gate terminal, the emitter-collector of the IGBT is turned on. The reason why the voltage is about 20 V is that a voltage exceeding the cutoff voltage can be supplied via the drive circuit 12 even if the cutoff voltage varies. That is, when driving the IGBT, it is important that the output voltage of the first DC power supply circuit 24, which is the power supply of the drive circuit 12 that supplies voltage to the IGBT, is larger than the cutoff voltage of the gate terminal.

Here, the operation of the drive circuit 12 briefly described above will be described in a little more detail. The drive circuit 12 uses the power supply voltage of the first DC power supply circuit 24 while the output of the pulse generator 18 formed by the microcomputer 15 is high, and applies a voltage to the gate terminal of the IGBT constituting the semiconductor switching element 5. Is applied, the IGBT collector-emitter is turned on, and while the pulse generator 18 is outputting low, the voltage of the IGBT gate terminal is set to 0 V, and I
Turn off the collector-emitter of the GBT. Note that this is an example, and the components constituting the push-pull circuit may be constituted by MOSFETs or the like. Further, the output voltage of the first DC power supply circuit 24 becomes the voltage set by the first DC power supply voltage setting unit 20 according to the ON time set by the ON time setting unit 17.

  The first DC power supply circuit 24 also supplies power for driving the cooling fan 27. Although not particularly shown in the present embodiment, the cooling fan 27 can be driven or stopped by turning on and off the transistors and MOSFETs as fan drive circuits by the microcomputer 15.

  The second DC power supply circuit 28 constitutes a constant voltage circuit by an emitter follower circuit composed of an NPN transistor 29, a constant voltage diode 30, a resistor 31, and a capacitor 32, and the output voltage of the first DC power supply circuit 24 is about 20V. The voltage is stepped down to 5V. The second DC power supply circuit 28 is a power supply for the microcomputer 15 and the zero voltage synchronization signal output circuit 33.

  The configuration of the second DC power supply circuit 28 is merely an example, and a dedicated power supply IC may be used to realize a constant voltage circuit that suppresses variations in output voltage, for example.

  The zero voltage synchronization signal output circuit 33 is not particularly shown, but the (u) pole of the AC power supply 7 is connected to the base terminal of the transistor through a resistor. The collector terminal of this transistor is connected to a second DC power supply circuit 28 generated from the AC power supply 7 via a switching power supply via a resistor, and (u) the potential of the pole is higher than the other pole. Is output to the microcomputer 15 and when it is low, it is output to the microcomputer 15. The microcomputer 15 inputs the output voltage of the input current detection circuit 13 after a predetermined time in synchronization with the output signal, and compares it with the input current set value set by the input current setting unit 19. Set the on time. The set on-time is updated at the next synchronization signal. The microcomputer 15 starts displaying the current time, processing related to rice cooking control, and the like in synchronization with the output signal of the zero voltage synchronization signal output circuit 33.

  The operation unit 34 includes a plurality of momentary switches. When each switch is pressed by the user, the microcomputer 15 detects that the switch has been pressed, and performs a predetermined rice cooking operation or heat retaining operation in accordance with each switch. The display unit 35 includes an LCD and a plurality of LEDs such as red, green, and orange. The microcomputer 15 sets the display content of the LCD and the LED to be lit according to the state of the rice cooker such as during rice cooking or heat insulation. The display content of the LCD includes a display of the current time, a display of the time until the end of cooking rice, a display of the heat retention elapsed time, and the like.

  FIG. 2 is a cross-sectional configuration diagram of a main part of the rice cooker according to the present embodiment. In order to simplify the drawing, lead wires for electrical connection and screws for fixing components are omitted. In FIG. 2, 41 is a body (main body) of a rice cooker. A lid 42 covering the upper surface of the body 41 is installed so as to be openable and closable. The housing portion 43 of the body 41 has the heating coil 2 disposed on the bottom and side portions thereof, and the ferrite cores 44 are disposed radially on the outer peripheral side of the heating coil 2. The heating coil 2 is a winding having a center substantially directly below the center of the bottom of the pan 1.

The pan 1 is formed of a magnetic material such as stainless steel, iron, or copper. The pan 1 has a flange 45 protruding outward from the upper end opening, and the flange 45 is placed in a state of being lifted from the upper end of the storage unit 43 so as to be detachably stored in the storage unit 43. Therefore, the pan 1 has a gap between the storage portion 43 during storage. The lid 42 is provided with a detachable lid heating plate 46. The lid heating plate 46 is made of a metal such as stainless steel. A lid heater 47 is built in the lid 32 and heats the lid heating plate 46. The lid heater 47 is not particularly shown in FIG.

  A first circuit board 48 includes a switch, an LCD, a microcomputer 15 and the like. Reference numeral 49 denotes a second circuit board on which an IGBT, a capacitor 4, a diode bridge 9, a choke coil 10, a capacitor 11 and the like constituting the semiconductor switching element 5 are mounted, although not particularly shown. Reference numeral 50 denotes a wind-up type power cord storage unit, which is electrically connected to the second circuit board 49 via a lead wire. The power cord storage unit 50 can wind up the power cord using a stopper and a spring.

  Reference numeral 51 denotes a temperature detector, which is composed of a thermistor, and is arranged substantially at the center of the bottom of the pan 1. Since the resistance value of the thermistor changes with temperature, a voltage dividing circuit is configured with this thermistor and a resistor having a predetermined resistance value, and the resistance value of the thermistor is analogized by supplying a predetermined voltage to both ends of the voltage dividing circuit. Can be converted to voltage. The microcomputer 15 shown in FIG. 1 estimates the temperature from this analog voltage using a built-in AD converter.

  The cooling fan 27 is composed of a fan motor in which a fan is attached to a rotor of a DC brushless motor. As shown in FIG. 1, the cooling fan 27 is supplied with power from the first DC power supply circuit 24 and is on / off controlled by the microcomputer 15.

  Although not shown, the first circuit board 48 and the second circuit board 49 are electrically connected by lead wires, and are turned on by the on-time setting unit 17 and the pulse generation unit 18 configured inside the microcomputer 15. The semiconductor switching element 5 is controlled to be turned on / off, and a high frequency current is supplied to the heating coil 2. The heating coil 2 generates an alternating magnetic field when a high-frequency current flows, and this alternating magnetic field causes an eddy current to flow through the pan 1 and the pan 1 generates heat.

  As mentioned above, the rice cooker of this Embodiment induction-heats the pan 1, and cooks the cooking thing in the pan 1 by heating. Here, the cooked product is rice before cooking rice and water or cooked rice.

  FIG. 3 shows a block diagram of the main part of the drive circuit 12. In FIG. 3, reference numeral 61 denotes an NPN transistor with a built-in resistor, which is turned on / off in response to a high / low signal from the microcomputer 15.

  An NPN transistor 62 is turned on when a base current is supplied from the first DC power supply circuit 24 through the resistor 63 when the resistor built-in NPN transistor 61 is turned off, and when the resistor built-in transistor 61 is turned on, the base terminal voltage is turned on. Turns off at about 0V. The resistor 64 is a resistor connected between the base and emitter of the NPN transistor 62, and prevents malfunction even when external noise enters the NPN transistor 62.

  Reference numeral 65 denotes an NPN power transistor for turning on the gate terminal of an IGBT which is an example of the semiconductor switching element 5, and 0.5A to 1A so that a charging current can be instantaneously supplied to the gate terminal of the IGBT which is the semiconductor switching element 5. A transistor capable of supplying instantaneous current is used. When the NPN transistor 62 is turned off, the base current is supplied from the first DC power supply circuit 24 via the resistor 66 and turned on. The NPN power transistor 65 may be a MOSFET. Since the MOSFET is a voltage driven type, it tends to be less affected by the resistor 66 than the power transistor, and is effective when used to save power in the drive circuit.

Reference numeral 67 denotes a current limiting resistor. When the NPN power transistor 65 is turned on, current is supplied from Vcc to the gate terminal of the IGBT which is the semiconductor switching element 5 through the current limiting resistor 67 and the NPN power transistor 65. The collector current of the NPN power transistor 65 is determined by the base current and the current amplification factor Hfe. In the present embodiment, considering the lower limit value of the current amplification factor Hfe as about 50, the resistor 66 is determined so that the collector current can sufficiently flow, and the current to the gate terminal of the IGBT is roughly determined by the current limiting resistor 67. I have to. Since the IGBT gate terminal of the semiconductor switching element 5 has a parasitic capacitance, the gate terminal voltage rises based on the current limiting resistor 67 and the time constant due to the parasitic capacitance value, and the cutoff voltage (threshold voltage) of the IGBT gate terminal is reduced. If it exceeds, the IGBT turns on.

  68 is a PNP power transistor for turning off the gate terminal of the IGBT, and has a capability of allowing an instantaneous current of about 0.5 A to 1 A to flow so that the charge charged in the parasitic capacitance of the gate terminal of the IGBT can be instantaneously extracted. When the NPN transistor 62 is turned on, the NPN power transistor 65 is turned off, and the base current of the PNP power transistor 68 is supplied from the gate terminal of the IGBT, so that the PNP power transistor 68 is turned on. As shown in FIG. 3, the PNP power transistor 68 has no base resistance, and the base current is determined by the collector current supply capability of the NPN transistor 62. The PNP power transistor 68 may be a MOSFET. Since the MOSFET is a voltage driven type, it tends to be less affected by the resistor 66 than the power transistor, and is effective when used to save power in the drive circuit.

  69 is a discharge resistor, and when the PNP power transistor 68 is turned on, the charge charged in the parasitic capacitance of the IGBT gate terminal is discharged from the IGBT gate terminal voltage via the PNP power transistor 68 and the discharge resistor 69, and finally about Set to 0V. The reason why the gate terminal voltage becomes 0 V as the discharge resistance 69 is reduced but becomes faster is because it is determined by the time constant of the parasitic capacitance between the discharge resistance 69 and the gate-emitter of the IGBT.

  The resistor 70 prevents the IGBT from malfunctioning due to external noise entering between the gate and the emitter of the IGBT when the PNP transistor 67 cannot be fully turned on until the first DC power supply circuit 24 rises to a predetermined voltage. This is for making the gate-emitter voltage the same potential.

  A constant voltage diode 71 clamps a voltage of a predetermined value or more. By making the voltage clamped by the constant voltage diode 71 smaller than the absolute maximum voltage rating between the gate and the emitter of the IGBT which is the semiconductor switching element 5, it is possible to prevent the breakdown of the IGBT. However, if the output voltage of the first DC power supply circuit 24 is set to a value smaller than the absolute maximum rated voltage between the gate and emitter of the IGBT and larger than the cutoff voltage (threshold voltage) between the gate and emitter of the IGBT. There is no need to force it. At this time, there is an advantage that the mounting area is reduced. When a protection circuit such as the constant voltage diode 71 is inserted, the first DC power supply circuit 24 is connected to another load (a circuit in which a lot of LEDs used for a cooling fan and display means are connected in series, although not particularly shown). It becomes a case where it raises to the required voltage.

  Reference numeral 72 denotes a parasitic capacitance between the gate and the emitter of the IGBT. This value generally varies depending on the rated current capacity, rated voltage and IGBT manufacturer of the IGBT.

  As described above, the drive circuit shown in FIG. 3 has the current limiting resistor 67 for limiting the charging current for charging the parasitic capacitance 72 between the gate and the emitter of the IGBT, and the parasitic capacitance between the gate and the emitter of the IGBT. The configuration includes a discharge resistor 69 for discharging the charge charged in 72, and the NPN power transistor 65 and the PNP power transistor 68 form a push-pull circuit configuration. However, this is only an example, and a dedicated IC may be provided with a current limiting resistor or a discharge resistor.

Further, the constant voltage diode 71 prevents the absolute maximum voltage rating of the gate terminal of the semiconductor switching element 5 from being exceeded, but this configuration is also an example, and there is a resistor between the first DC power supply circuit 24 and the drive circuit. A circuit that limits the voltage with a constant voltage diode may be provided, or a sufficient supply current may be maintained while a constant voltage circuit is configured with an emitter follower circuit of a transistor like the second DC power supply circuit. .

  FIG. 4 is a graph showing the relationship between the ON time of the semiconductor switching element 5 of the rice cooker of the present embodiment and the peak value of the collector current flowing through the semiconductor switching element 5. In the present embodiment, Ton1 is set to the initial value of the on-time of the semiconductor switching element 5. As shown in FIG. 4, the peak value of the collector current increases as the ON time increases. That is, when the on-time is short like Ton1, even if the voltage at the gate terminal of the IGBT constituting the semiconductor switching element 5 is lowered to increase the collector-emitter voltage, the switching loss does not increase so much.

  FIG. 5 is a graph showing the relationship between the input current supplied from the AC power supply 7 of the rice cooker of the present embodiment and the peak value of the collector current flowing through the semiconductor switching element 5. As shown in FIG. 4, the collector current increases as the input current increases. That is, when the input current is as small as the third input current set value 23, even if the voltage at the gate terminal of the IGBT constituting the semiconductor switching element 5 is lowered to increase the collector-emitter voltage, the switching loss is very large. Don't be.

  FIG. 6 is a graph showing the relationship between the collector-emitter voltage and the collector current of the IGBT constituting the semiconductor switching element 5 of the rice cooker of the present embodiment. FIG. 6A is a graph when the gate terminal voltage of the IGBT is 10V. FIG. 6B is a graph when the gate terminal voltage of the IGBT is 15V. FIG. 6C is a graph when the gate terminal voltage of the IGBT is 20V. As shown in FIG. 6, the collector-emitter voltage and the collector current vary depending on the gate terminal voltage.

  Considering that the peak value of the collector current increases as the ON time of the semiconductor switching element 5 becomes longer as shown in FIG. 4, the voltage at the gate terminal of the IGBT constituting the semiconductor switching element 5 is lowered when the ON time is shorter. Thus, it can be determined that the switching loss does not increase so much even if the collector-emitter voltage increases.

  In consideration of the fact that the collector current increases as the input current increases as shown in FIG. 5, when the input current is small, the voltage at the gate terminal of the IGBT constituting the semiconductor switching element 5 is lowered to reduce the collector-emitter voltage. Even if it rises, it can be judged that the switching loss does not become so large.

FIG. 7 shows operation waveforms of main parts when the microcomputer 15 controls the semiconductor switching element 5 on and off at Ton1 (initial value) in the induction heating rice cooker shown in FIGS. 1 and 3. FIG. 7A shows an on / off signal output from the microcomputer 15. FIG. 7B shows the gate-emitter voltage of the IGBT which is the semiconductor switching element 5. FIG. 7C shows the collector current of the IGBT that is the semiconductor switching element 5. FIG. 7D shows the collector-emitter voltage of the IGBT which is the semiconductor switching element 5. The voltage waveform between the gate and emitter of the IGBT shown in FIG. 7B is maintained until a voltage of about 5V is sufficiently charged in the parasitic capacitance and the collector-emitter of the IGBT can be turned on. When the charge is sufficiently charged and the IGBT is turned on, the gate-emitter voltage starts increasing again according to the time constant of the parasitic capacitance 72 and the current limiting resistor 67. In order to obtain such a waveform, it is desirable that the output voltage of the first DC power supply circuit 24 is at least twice the cutoff voltage (threshold voltage) of the gate terminal of the IGBT. The gate-emitter voltage increases because it is more than twice, and even if a large amount of collector current flows, the collector-
The effect is that the emitter-to-emitter voltage remains low.

  FIG. 8 shows the operation waveform of the main part when the microcomputer 15 performs on / off control of the semiconductor switching element 5 with Ton1 (initial value) when the drive circuit of the induction heating rice cooker of the present embodiment is not controlled. Is shown. FIG. 7A shows an on / off signal output from the microcomputer 15. FIG. 7B shows the gate-emitter voltage of the IGBT which is the semiconductor switching element 5. FIG. 7C shows the collector current of the IGBT that is the semiconductor switching element 5. FIG. 7D shows the collector-emitter voltage of the IGBT which is the semiconductor switching element 5. In FIG. 8, since the gate-emitter voltage of the IGBT is large, the collector-emitter voltage can be reduced even if a large amount of collector current flows, but on the other hand, the collector current flows too much, and the absolute maximum rating of the collector current of the IGBT is reduced. It is necessary to consider exceeding. Further, since the collector-emitter voltage does not fall below a predetermined value due to the characteristics of the IGBT, the turn-on loss of the IGBT is greatly affected by the collector current.

  7 and 8, since the collector-emitter voltage Von of the IGBT which is the semiconductor switching element 5 is generated at the turn-on time t1, the inrush current Ion flows in the collector current of the IGBT at the turn-on time. The semiconductor switching element 5 generates heat due to Von and Ion at this time, but in the case of an induction heating rice cooker which is one of the present embodiments, as shown in FIG. 7, the gate-emitter voltage of the IGBT. In accordance with the relationship between the gate-emitter voltage and the collector current graph shown in FIG. 6, the inrush current can be smaller than when the gate-emitter voltage is 20 V as shown in FIG.

  FIG. 9 shows an operation waveform of the main part when the microcomputer 15 performs on / off control of the gate terminal of the IGBT which is the semiconductor switching element 5 in the induction heating rice cooker of the present embodiment. FIG. 9A shows an on / off signal output from the microcomputer 15. FIG. 9B shows the gate-emitter voltage of the IGBT which is the semiconductor switching element 5. FIG. 9C shows the collector current of the IGBT that is the semiconductor switching element 5. FIG. 9D shows the collector-emitter voltage of the IGBT that is the semiconductor switching element 5.

  As shown in FIG. 9B, the collector current of the IGBT also gradually increases due to the inductance component of the heating coil 2. In the case of the induction heating type rice cooker of the present embodiment, the time change amount (dIc / dt) of the collector current is Vc2 applied to both ends of the heating coil 2 when the IGBT is turned on, and the inductance of the heating coil 2 is L1. Then, the relational expression (Vc2 / L1) = (dIc / dt) is obtained. On the other hand, the gate-emitter voltage of the IGBT increases according to the time constant of the current limiting resistor 67 of the drive circuit 12 and the parasitic capacitance 72 of the IGBT, and finally the output voltage of the first DC power supply circuit 24 is 20V. Approaching.

  As described above, the gate-emitter voltage of the IGBT increases according to the time constant of the current limiting resistor 67 and the parasitic capacitance 72 of the IGBT. Therefore, the collector-emitter voltage of the IGBT decreases according to the gate-emitter voltage. Therefore, even if the collector current of the IGBT increases according to the above relational expression, the on-loss of the IGBT does not need to increase.

FIG. 10 shows a time chart of the temperature detection unit of the rice cooker of the present embodiment, an input current setting value set by the input current setting unit, and a time chart of the output voltage of the first DC power supply circuit 24. Fig.10 (a) has shown the time chart of the pan bottom temperature which the temperature detection part detected. FIG. 10B shows a time chart of the input current set value set by the input current setting unit 19. In the time chart of the input current set value, the vertical axis indicates the input current set value, and the horizontal axis indicates a period in which the semiconductor switching element is turned on / off with this input current set value and a high-frequency current is supplied to the heating coil. FIG. 10C shows a time chart of the ON time of the semiconductor switching element 5 output from the microcomputer 15.

  Operation | movement of the rice cooker of this Embodiment is demonstrated using FIGS.

  In S0 of FIG. 10, among the plurality of momentary switches (operation unit 31) mounted on the first circuit board 48 arranged in the lid of the rice cooker shown in FIG. When pressed, the microcomputer 15 shown in FIG. 1 detects the signal of the operation unit 34, and the sequence setting unit 20 sets the rice cooking sequence and starts the rice cooking sequence.

  The rice cooking sequence is composed of a plurality of subsequences. In this Embodiment, it is comprised by the pre-cooking process, the rice cooking amount determination process, the boiling maintenance process, and the additional cooking process.

  The period from S0 to S3 corresponds to the pre-cooking process. In the pre-cooking process, the input current setting unit 19 receives the signal from the sequence setting unit 20 and sets the second input current set value 22 (Iin2) until the water is easily absorbed by the rice during the period from S0 to S3. To do.

  Thereafter, the on-time of the semiconductor switching element 5 is set by setting Ton1 as an initial value and comparing the output voltage of the input current detection circuit 13 with the second input current setting value 22 to set the on-time. The increase time or decrease time of the on time is set so as not to exceed a predetermined time. The reason is that when a sudden current change occurs, a sharp power is applied to the pan 1 by this amount of current change, and a sound is produced. Gradually increases the on-time. When the output voltage of the input current detection circuit 13 exceeds the output voltage corresponding to the second input current setting value 22 in S1, the on-time setting unit 17 sets the output voltage to the second voltage. The on-time is adjusted so that the input current becomes the second input current set value 22. The timing at which the microcomputer 15 reads the output voltage of the input current detection circuit 13 and adjusts the on-time is performed after a predetermined time, triggered by the low-to-high edge of the zero voltage synchronization signal output circuit 33. ing. This predetermined time is such a time that the instantaneous value of the output voltage of the input current detection circuit 13 peaks. This corresponds to the vicinity of the peak of the AC power supply 7.

  The operation at this time will be repeated in the future, and the description of this operation will be omitted in FIG. At this time, the gate-emitter voltage of the IGBT which is the semiconductor switching element 5 is a waveform that rises due to the time constant of the current limiting resistor 67 and the parasitic capacitance 72 of the IGBT, as shown in FIGS. It becomes.

  The conduction ratio at which the semiconductor switching element 5 operates by setting the second input current set value 22 is 10 seconds / 16 seconds. When restarting at 10/16 seconds, the on-time setting unit 17 of the microcomputer 15 sets the on-time to the initial value Ton1 again, and sets the output voltage of the input current detection circuit 13 and the second input current setting. The values 22 are compared, and the ON time is gradually increased so that the voltage corresponding to the second input current setting value 22 is obtained.

  When the temperature detection unit 51 shown in FIG. 2 detects 60 degrees in S2, the microcomputer 15 conducts the output voltage of the input current detection circuit 13 at the second input current set value 22 according to the preprogrammed contents. The ratio is made variable, and a temperature of about 60 degrees is controlled so as to be maintained for a period until S3.

The period from S3 to S5 corresponds to the cooking amount determination process. In the rice cooking amount determination step, the input current setting unit 19 sets the set value of the input current to the first input current set value 21 (Iin1) according to the output of the sequence setting unit 20. The on-time setting unit 17 first sets the on-time to the initial value Ton1.
After that, the output voltage of the input current detection circuit 13 and the first input current set value 21 are compared, and a high frequency current is supplied to the heating coil 2 while increasing the ON time.

  In S4, when the output voltage of the input current detection circuit 13 exceeds the voltage corresponding to the first input current set value 21, the on-time setting unit 17 compares the output voltage with the second input current set value 22. The on-time is adjusted so that the input current becomes the second input current set value 22. The timing at which the microcomputer 15 reads the output voltage of the input current detection circuit 13 and adjusts the on-time is performed after a predetermined time, triggered by the low-to-high edge of the zero voltage synchronization signal output circuit 33. ing. This predetermined time is such a time that the instantaneous value of the output voltage of the input current detection circuit 13 peaks. This corresponds to the vicinity of the peak of the AC power supply 7.

  Since the first input current set value 21 (Iin1) is larger than the second input current set value 22 (Iin2), the current supplied to the heating coil 2 also increases and the induction heating amount also increases. That is, the temperature of the pan also rises rapidly. In the present embodiment, from the elapsed time (time from S3 to S5) until the temperature detection unit 51 detects 100 degrees after driving the heating coil 2 with the first input current set value 21 being started. The microcomputer 15 determines the amount of rice cooking, and sets the conduction ratio for driving the heating coil 2 in the subsequent boiling maintenance process and the conduction ratio for driving the heating coil 2 in the additional cooking process. In addition, in the induction heating type rice cooker of this Embodiment, although the temperature detection part 51 determines the amount of rice cooking by the time until the bottom temperature of the pan 1 becomes 100 degreeC, this is an example and it is an inner side of a lid | cover. Alternatively, another temperature detection unit may be provided, and the amount of cooked rice may be determined based on the elapsed time until the temperature of the temperature detection unit exceeds a predetermined temperature.

  The period from S5 to S6 corresponds to the boiling maintenance process. In the boiling maintenance process, the input current setting unit 19 sets the second input current setting value 22 (Iin2) according to the output of the sequence setting unit 20, and the on-time setting unit 17 determines the output voltage of the input current detection circuit 13. The ON time of the semiconductor switching element 5 is set by comparing the second input current set value 22, and the pan 1 is heated by supplying a high-frequency current to the heating coil 2. In the rice cooker of the present embodiment, heating coil 2 is driven at a conduction ratio of 10 seconds / 16 seconds in accordance with instructions from sequence setting unit 20. When the heating coil 2 is driven to continue induction heating of the pan 1, the water remaining in the pan 1 also evaporates, and the temperature of the pan 1 exceeds 100 degrees and reaches 130 degrees in S6. When the temperature detection unit 51 detects 130 degrees, the microcomputer 15 turns off the semiconductor switching element 5, stops driving the heating coil 2, ends the boiling maintenance process, and shifts to the cooking process.

  The period from S6 to S7 corresponds to the cooking process. In the additional cooking process, the input current setting unit 19 sets the second input current set value 22 according to the output of the sequence setting unit 20. The on-time setting unit 17 compares the second input current set value 22 and the output voltage of the input current detection circuit 13 to set the on-time of the semiconductor switching element 5. The pulse generator 18 outputs the time set by the on-time setting unit 17 as a high pulse with the synchronization signal of the synchronization signal generation circuit 16 as a trigger, and finally the output voltage of the input current detection circuit 13 becomes the second input current setting. Control is performed so that the value is 22. The sequence setting unit 20 sets the conduction ratio of 2 seconds / 16 seconds for the period of operation at the second input current set value 22. The sequence setting unit 20 of the microcomputer 15 measures the elapsed time from S6, and when S7 is reached, the rice cooking sequence is terminated, the boiled buzzer is notified, and the LED indicating the meaning during rice cooking constituting the display unit 35 is turned off. It is displayed that cooking rice is finished.

At the same time, in S7, the sequence setting unit 20 constituting the microcomputer 15 turns on the LED indicating the meaning during the heat insulation constituting the display unit 35, and starts the heat insulation sequence. The input current setting unit 19 sets the input current set value to the third input current set value 23 (Iin3) in accordance with an instruction from the sequence setting unit 20. Thereafter, the semiconductor switching element 5 is turned off until the temperature of the temperature detection unit 51 becomes lower than a predetermined temperature. When the temperature becomes lower than the predetermined temperature, the semiconductor switching element 5 is turned on / off again, and the output voltage of the input current detection circuit 13 is turned on. Is controlled to be the third input current set value 23. At this time, the output voltage of the input current detection circuit 13 is equal to or lower than the output voltage corresponding to the third input current set value 23.

  By controlling the semiconductor switching element 5 to be turned on / off so that the third input current set value 23 is obtained, the pan 1 is inductively heated to maintain this temperature. The heat retention sequence does not require the energy to make rice as in the rice cooking sequence, and it is sufficient if there is enough energy to maintain the rice temperature at a constant temperature, so the input current can be smaller than in the rice cooking sequence. It is.

  The heat retention sequence is stopped when the user presses a switch indicating cancellation on the operation unit 34 or when a predetermined time elapses. The microcomputer 15 determines that it is in a standby state, and turns off the semiconductor switching element 5.

  As described above, in FIG. 10, the input current set values are the first input current set value 21 (Iin1), the second input current set value 22 (Iin2), and the third input current set value 23 (Iin3). ) And multiple settings are made so that the rice cooking sequence and the heat retention sequence can be executed.

  When the semiconductor switching element 5 is controlled to be turned on / off so as to have these input current set values, the initial value of the on-time at the start-up of the semiconductor switching element 5 is set to Ton1, and the on-time is gradually lengthened. The on-time setting unit 18 adjusts the on-time so that the output voltage of the current detection circuit 13 becomes the input current set value set by the input current setting unit 19. As shown in FIG. 5, when the input current is set to a predetermined value, the collector current of the IGBT that is the semiconductor switching element 5 is also controlled to be substantially constant.

  Then, by making the output voltage of the first DC power supply circuit 24 sufficiently higher than the threshold voltage of the semiconductor switching element 5 (in the present embodiment, it is about 2 to 3 times), it is shown in FIGS. As described above, since the gate-emitter voltage of the IGBT which is the semiconductor switching element 5 can be increased as the length of the on-time is increased, the inrush current of the IGBT generated at the time of start-up shortens the on-time. Can be reduced. The fact that the inrush current can be reduced makes it possible not to exceed the absolute maximum rating of the instantaneous current of the IGBT, and it is possible to suppress the current breakdown of a semiconductor switching element such as an IGBT.

  Even if the collector current of the IGBT is increased, the gate-emitter voltage is increased and the collector-emitter voltage is decreased by increasing the ON time, so that the switching loss of the IGBT which is the semiconductor switching element 5 can be reduced. .

  In the present embodiment, the output voltage of the first DC power supply circuit is about 20V, but it may be about 15V. As shown in FIG. 6, when the IGBT gate-emitter voltage is 20V and 15V, there is almost no difference in the collector-emitter voltage depending on the magnitude of the collector current, as in the second DC power supply circuit. The loss of the circuit receiving power supply from the first DC power supply circuit can be reduced. Even if it is 15 V, the cutoff voltage (threshold voltage) is 6 V in the case of an IGBT which is an example of the semiconductor switching element of the induction heating rice cooker of this embodiment, so that the parasitic capacitance between the gate and the emitter of the IGBT is charged. It can be said that the output voltage is sufficient.

Further, in the present embodiment, the configuration of the drive circuit 12 is as shown in FIG. 3, but this may be changed to an IGBT drive IC. In this case, the current limiting resistor for charging the parasitic capacitance between the gate and the emitter of the IGBT needs to be set to some extent according to the capability of the driving IC, but the output voltage of the first DC power supply circuit is set to the IGBT The same effect can be obtained if the power is supplied to the driving IC by 2 to 3 times the cut-off voltage (threshold voltage).

(Embodiment 2)
FIG. 11: is a principal part block diagram of the induction heating type rice cooker in the 2nd Embodiment of this invention. In FIG. 11, the control unit 81 includes a voltage detection circuit 82, a microcomputer 83, a synchronization signal generation circuit 16, a temperature detection circuit for detecting the temperature of the pan 1, although not particularly shown.

  The voltage detection circuit 82 detects a value corresponding to a voltage between the connection portion (CE) of the heating coil 2 and the semiconductor switching element 5 and the ground. By detecting this voltage, a voltage corresponding to the voltage across the heating coil 2 can be detected. This is because the voltage between (CE) and the ground is the sum of the voltage at the connection (C2) between the capacitor 11 and the heating coil 2 and the voltage at the heating coil 2, and the voltage at the (C2) portion rectifies the AC power supply 7. This is because the voltage is low. Although not specifically shown, the voltage detection circuit 82 includes a resistance voltage dividing circuit and an emitter follower circuit for holding a peak value of the voltage output from the resistance voltage dividing circuit, and outputs an analog voltage to the AD port of the microcomputer 83. At this time, the voltage detection circuit 82 sets the voltage dividing ratio of the resistance voltage dividing circuit so that the output voltage does not exceed 4V.

  The microcomputer 83 includes an on-time setting unit 84, a pulse generation unit 18, a voltage setting unit 85, and a sequence setting unit 20 including a counter and a memory.

  The on-time setting unit 84 compares the output of the voltage setting unit 85 and the output of the voltage detection circuit 82 and sets the on-time of the semiconductor switching element 5. On-time setting unit 84 The on-time is set within the range of the minimum on-time Ton1 and the maximum on-time Ton4. In the present embodiment, when the inverter circuit 3 is activated, it starts from Ton1 as an initial value and gradually increases the on-time.

  The voltage setting unit 85 stores a first voltage setting value 86, a second voltage setting value 87, and a third voltage setting value 88. The relationship between the voltage setting values is such that the first voltage setting value 86> the second voltage setting value 87> the third voltage setting value 88.

  Although not shown in particular, the resonance voltage generated by the resonance of the heating coil 2 and the capacitor 4 when the semiconductor switching element 5 is turned off, that is, the voltage across the heating coil 2 is the current flowing through the semiconductor switching element 5 or the AC power supply 7. If the voltage waveform is constant, there is a proportional relationship with the input current supplied from the AC power supply 7.

  That is, if the on-time setting unit adjusts the on-time so that the voltage setting value set by the voltage setting unit 85 and the voltage output from the voltage detection circuit 82 are the same, the desired power can be obtained by the heating coil 2. it can.

  Other configurations are the same as those of the first embodiment, and a description thereof is omitted here.

  The operation of the induction heating rice cooker of FIG. 8 will be described.

As in the first embodiment, the sequence setting unit 20 sets the rice cooking sequence by operating the operation unit 34, and the voltage setting unit 85 sets the second voltage set value 87 by a signal from the sequence setting unit 20. Then, the on-time setting unit 84 first sets the on-time of the semiconductor switching element 5 to the initial value Ton1, and then increases the on-time of the semiconductor switching element 5 so as to become the second voltage setting value 87. Go.

  The on-time setting unit 84 increases the on-time of the semiconductor switching element 5 while comparing the output value of the voltage detection circuit 82 with the second voltage setting value 87, and the output voltage of the voltage detection circuit 82 becomes the second voltage. When the set value 87 is exceeded, the output voltage of the voltage detection circuit 82 is shortened to become the second voltage set value 87.

  As described above, the value corresponding to the voltage across the heating coil is detected by the voltage detection circuit, and the on-time setting unit is turned on so that the voltage setting value set by the voltage setting unit is set according to the rice cooking sequence or the heat retention sequence. By adjusting the time, the necessary rice cooking power and heat insulation power can be supplied without detecting the input current. With this configuration, a current transformer or the like for detecting the input current is not required, the mounting area can be reduced, for example, the second circuit board shown in FIG. 2 can be reduced, and the rice cooker can be downsized. Can be realized.

  Further, when the voltage corresponding to the voltage across the heating coil is low, a turn-on voltage of the semiconductor switching element may be generated as shown in FIG. At this time, since the on-time is short, the gate-emitter voltage of the IGBT, which is a semiconductor switching element, is reduced, and the collector current of the IGBT is limited as shown in FIG. 6, so that the inrush current can be reduced. , Turn-on loss can be reduced.

  Further, when the on-time is long, the current flowing through the heating coil also increases, and the voltage corresponding to the voltage across the heating coil also increases. When the current flowing through the heating coil increases, the collector current of the IGBT that is a semiconductor switching element also increases. However, as shown in FIG. 9, the gate-emitter voltage increases due to the charging current charged from the first DC power supply circuit to the gate terminal of the semiconductor switching element via the drive circuit, and the collector-emitter voltage of the IGBT is increased. Gradually decreases, and the steady-state ON loss of the IGBT can be reduced.

  As mentioned above, the rice cooker concerning this invention supplies the electric power of a drive circuit from a 1st DC power supply circuit so that the voltage applied to the gate terminal of a semiconductor switching element can be adjusted with time to turn on a gate terminal. Since it is doing, the induction heating type rice cooker with a small loss of a semiconductor switching element can be provided. Therefore, it can be applied not only to home use but also to uses such as rice cookers for business use.

DESCRIPTION OF SYMBOLS 1 Pan 2 Heating coil 3 Inverter circuit 5 Semiconductor switching element 7 AC power supply 8 Rectifier circuit 12 Drive circuit 13 Input current detection circuit 14 Control part 21 1st input current setting value 22 2nd input current setting value 23 3rd input Current setting value 24 First DC power supply circuit 28 Second DC power supply circuit 81 Control unit 82 Voltage detection circuit 85 Voltage setting unit 86 First voltage setting value 87 Second voltage setting value 88 Third voltage setting value

Claims (5)

Rectification and heating coil for induction heating the pan, and a capacitor connected in parallel or connected in series with said heating coil, connected to the heating coil, conducting the heating coil, an inverter circuit having a semiconductor switching element for interrupting an AC power source A rectifier circuit that supplies power to the inverter circuit; a drive circuit that turns on and off the gate terminal of the semiconductor switching element; a control unit that controls on / off of the semiconductor switching element via the drive circuit; and Yes a first DC power supply circuit for converting supplied power supply voltage than the first DC power supply circuit, a second DC power supply circuit to a voltage lower than the output voltage of the first DC power supply circuit, to And
It said first direct current power supply circuit is configured to output at least twice the voltage than the threshold voltage of the gate terminal of the semiconductor switching element,
The control unit is supplied with power supply voltage from the second DC power supply circuit, the driving circuit is powered from the first DC power supply circuit,
The control unit, by increasing the length of time of the output ON signal, rice cookers which the driving circuit is adjusted by increasing the voltage output to the gate terminal of the semiconductor switching element. Cooker according to claim 1, wherein the driving circuit has an upper limit value to the voltage output to the gate terminal of the semiconductor switching element. It has an input current detection circuit for detecting a value corresponding to the input current supplied to the rectifying circuit from the AC power source, wherein the control unit includes a microcomputer which stores the warmth sequence cooking sequence, the microcomputer in response to said cooking sequence and the warmth sequence has a predetermined input current set value, by comparing the output value of the input current detecting circuit and the input current set value, the output value is the input of the input current sense circuit The rice cooker of Claim 1 or 2 which sets ON time so that it may become an electric current setting value. Has a voltage detection circuit for detecting a voltage corresponding to the voltage across the heating coil, wherein the control unit includes a microcomputer which stores a cooking sequence and warmth sequence, the microcomputer in the thermal insulation sequence and the cooking sequence depending has a predetermined voltage setting value, by comparing the output value of the voltage detection circuit and the voltage setting value, wherein the output value of said voltage detection circuit is set to the on-time so that the voltage setting value Item 9. A rice cooker according to item 1 or 2 . The control unit has an initial value of the on-time, the initial value is a value exceeding the threshold voltage of the gate terminal voltage of the semiconductor switching elements, when starting the heating coil, the ON time of the semiconductor switching elements The rice cooker according to claim 3 or 4 , wherein the initial value is set.

Images